The present invention relates to magnetic materials. In particular, the invention relates to magnetic materials used in magnetic resonance imaging system shims.
Magnetic Resonance Imaging (MRI) systems typically include a superconducting magnet which generates a primary magnetic field within an imaging volume. Inhomogeneities in the primary magnetic field are a result of manufacturing tolerances for the magnet, and equipment and site conditions. Magnetic field inhomogeneities distort the position information in the imaging volume and degrade the image quality. The imaging volume must have a low magnetic field inhomogeneity for high quality imaging. Shimming is a known technique for reducing the inhomogeneity of the primary magnetic field. The primary magnetic field can be pictured as a large constant field with small inhomogeneous field components superimposed on the constant field. If the negative of the inhomogeneous components of the field can be generated, the net field will be made uniform and the magnet is then said to be shimmed.
It is known to use active or passive shims for reducing the magnetic field inhomogeneity. Active shimming is accomplished using resistive shim coils to generate magnetic fields designed to cancel out the inhomogeneous field components. Passive shimming is accomplished using shims comprised of ferromagnetic materials such as carbon steel. A magnetic field arising from an induced magnetic dipole of the shim is used to cancel out the inhomogeneous field components. The number, mass, and position of the shims are determined by known shimming techniques. The shims are contained in a shim assembly located near a gradient coil structure that generates the x, y, and z gradient magnetic fields used for MRI. The shim assembly is in thermal contact with the outer section of the gradient coil structure. Pulsing the gradient coils results in heat generation due to joule losses. A portion of the heat generated is transferred to the shim assembly causing an increase in the temperature of the shims. The higher temperature reduces the magnetization of the shim material, and weakens the magnetic field the shims produce. This results in an increase in the magnetic field inhomogeneity.
The concept of reduction of the magnetic field produced by a ferromagnetic shim element with increasing temperature is illustrated in FIG. 1 and table I below. A ferromagnetic material has a spontaneous magnetic moment and a magnetization which is defined as the magnetic moment per unit volume. The magnetic moments in a ferromagnetic material are aligned in the same direction. Above a temperature called the Curie temperature (Tc), spontaneous magnetic moments and magnetization vanish. FIG. 1 shows the change in a relative magnetization of nickel as a function of temperature. Relative magnetization is shown as the ratio of the magnetization at a temperature, T to the magnetization at about 0 K. The horizontal axis represents the ratio of the temperature, T to the Curie temperature, Tc. As the temperature increases, the magnetization decreases until it vanishes at the Curie temperature. Examples of ferromagnetic materials other than nickel include iron, cobalt, iron alloys, cobalt alloys, nickel alloys, and intermetallic compounds such as MnAs and MnBi. Table 1 lists the magnetization at or near room temperature and at about 0 K. As can be seen from Table 1, the value for magnetization at or near room temperature is lower than that at about 0 K.
TABLE 1MaterialBi/4π @ room temperature, GaussBi/4π @ 0 K, GaussFe17071740Co14001446Ni485510MnAs670870MnBi620680